Forming structures using aerosol jet® deposition

ABSTRACT

Method and apparatus for direct writing of passive structures having a tolerance of 5% or less in one or more physical, electrical, chemical, or optical properties. The present apparatus is capable of extended deposition times. The apparatus may be configured for unassisted operation and uses sensors and feedback loops to detect physical characteristics of the system to identify and maintain optimum process parameters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/720,573, entitled “Aerodynamic Jetting of BlendedAerosolized Materials”, filed on Mar. 9, 2010, issuing on Aug. 5, 2014as U.S. Pat. No. 8,796,146, which application is a continuationapplication of U.S. patent application Ser. No. 11/302,481, entitled“Aerodynamic Jetting of Aerosolized Fluids for Fabrication of PassiveStructures”, filed on Dec. 12, 2005, which issued as U.S. Pat. No.7,674,671, which application claims the benefit of the filing of U.S.Provisional Patent Application Ser. No. 60/635,848, entitled“Solution-Based Aerosol Jetting of Passive Electronic Structures” filedon Dec. 13, 2004. The specification and claims of these applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The present invention relates generally to the field of directdeposition of passive structures. More specifically, the inventionrelates to the field of maskless, precision deposition of mesoscalepassive structures onto planar or non-planar targets, with an emphasison deposition of precision resistive structures.

2. Background Art

Note that the following discussion refers to a number of publicationsand references. Discussion of such publications herein is given for morecomplete background of the scientific principles and is not to beconstrued as an admission that such publications are prior art forpatentability determination purposes.

Various methods for deposition of passive structures exist, however,thick film and thin film methods have played a dominant role in thedeposition of passive structures, including but not limited to resistorsor capacitors, onto various electronic and microelectronic targets. Byway of example, the thick film technique typically uses ascreen-printing process to deposit electronic pastes with linewidths assmall as 100 microns. Thin film methods for the printing of electronicstructures include vapor deposition techniques, such as chemical vapordeposition and laser-assisted chemical vapor deposition, as well asphysical deposition techniques, such as sputtering and evaporation.

U.S. Pat. No. 4,938,997 discloses a method for the fabrication of thickfilm resistors on ceramic substrates, with tolerances consistent withthose required for microelectronic circuitry. In this method, aruthenium-based resistor material is screen printed onto the substrateand fired at temperatures in excess of 850° C. U.S. Pat. No. 6,709,944discloses a method for fabrication of passive structures on flexiblesubstrates by using ion bombardment to activate the surface of asubstrate such as polyimide, forming a graphite-like carbon region thatmay be combined with another deposited material—such as titanium—to forma passive structure. U.S. Pat. No. 6,713,399 discloses a method for thefabrication of embedded resistors on printed circuit boards. The methoduses a thin film process to form embedded passive structures in recessesthat have been formed in a conductive layer. The method of U.S. Pat. No.6,713,399 discloses a process that eliminates the high resistancevariation seen in polymer thick film embedded resistors.

While thick film and thin film methods of passive structure fabricationare well-developed, these processes may be unsuitable for certaindeposition applications. Some disadvantages of thick film processes arethe relatively large minimum linewidths that are characteristic of thetechnique, the need for mask utilization, and the need forhigh-temperature processing of the deposited material. The disadvantagesof typical thin film processes include the need to use masks, vacuumatmospheres, and multi-step photolithographic processes.

In contrast with conventional methods for deposition of passivestructures, the M³D® process is a direct printing technique that doesnot require the use of vacuum chambers, masks, or extensivepost-deposition processing. Commonly-owned International PatentApplication Number PCT/US01/14841, published as WO 02/04698 andincorporated herein by reference, discloses a method for using anaerosol jet to deposit passive structures onto various targets, butgives no provision for lowering the tolerance of deposited structures tolevels that are acceptable for manufacturing of electronic components.Indeed, the use of a virtual impactor in the invention disclosed thereineventually leads to failure of the system due to the accumulation ofparticles in the interior of the device. As a result, the maximumruntime before failure of the previously disclosed system is 15 to 100minutes, with the electrical tolerances of deposited structures ofapproximately 10% to 30%.

Contrastingly, the present invention can deposit passive structures withconductance, resistance, capacitance, or inductance values withtolerances of less than 5%, and runtimes of several hours.

SUMMARY OF THE INVENTION

The present invention is an apparatus for depositing a passive structurecomprising a material on a target, the apparatus comprising an atomizerfor forming an aerosol comprising the material and a carrier gas, anexhaust flow controller for exhausting excess carrier gas, a depositionhead for entraining the aerosol in a cylindrical sheath gas flow, apressure sensing transducer, a cross connecting the atomizer, thedeposition head, the exhaust flow controller, and the transducer,wherein the tolerance of a desired property of the passive structure isbetter than approximately 5%. The deposition head and atomizer arepreferably connected to the cross at inlets opposite each other. Theexhaust flow controller preferably exhausts excess carrier gas at adirection perpendicular to an aerosol direction of travel through thecross. The exhaust flow controller preferably reduces the carrier gasflowrate.

The apparatus preferably further comprises a processor for receivingdata from the transducer, the processor determining if a leak or clog ispresent in the apparatus. In such case the apparatus preferably furthercomprises a feedback loop for automatically purging the apparatus if aclog is detected or automatically ceasing operation of the apparatus ifa leak is detected. The apparatus preferably further comprises a laserwhose beam passes through the flowing aerosol and a photodiode fordetecting scattered light from the laser. The laser beam is preferablyperpendicular to the flow direction of the aerosol and the photodiode ispreferably oriented orthogonally to both the laser beam and the flowdirection. The photodiode is preferably connected to a controller forautomatically controlling the atomizer power.

The invention is also a method of depositing a passive structurecomprising a material on a target, the method comprising the steps of:atomizing the material; entraining the atomized material in a carriergas to form an aerosol; removing excess carrier gas from the aerosol viaan opening oriented perpendicularly to a flow direction of the aerosol;monitoring a pressure of said aerosol; surrounding the aerosol with asheath gas; and depositing the material on the target; wherein atolerance of a desired property of the passive structure is better thanapproximately 5%. The method preferably further comprises the steps ofdetermining the existence of a leak or clog based on a value of thepressure, and automatically purging the system if a clog exists orautomatically ceasing operation if a leak exists. The method preferablyfurther comprises the steps of shining a laser beam into the aerosol andmeasuring scattered light from the laser beam. The measuring step ispreferably performed by a detector oriented orthogonally to both thelaser beam and a flow direction of the aerosol. The method preferablyfurther comprises the step of varying the power used in the atomizingstep based on an amount of scattered light detected in the measuringstep.

The method preferably further comprises the step of processing thematerial, the processing step preferably selected from the groupconsisting of humidifying the aerosol, drying the aerosol, heating theaerosol, heating the deposited material, irradiating the depositedmaterial with a laser beam, and combinations thereof. Irradiating thedeposited material with a laser beam preferably enables a linewidth ofthe deposited material to be as low as approximately 1 micron.Irradiating the deposited material with a laser beam preferably does notraise an average temperature of the target to above a damage threshold.

An object of the present invention is to pre-process a material inflight and/or post processing treatment the material after itsdeposition on a target resulting in a physical and/or electricalproperty having a value near that of a bulk material.

Another object of the present invention is to provide a depositionapparatus which is capable of long runtimes.

An advantage of the present invention is that deposited passivestructures have conductance, resistance, capacitance, or inductancevalues with tolerances of less than 5%.

Other objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

A BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate several embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating a preferred embodiment of the invention and are not to beconstrued as limiting the invention. In the drawings:

FIG. 1a is a schematic of the embodiment of the preferred M³D® apparatusof the present invention capable of extended runtimes and depositingpassive structures with tolerances below 5%.

FIG. 1b shows the general embodiment of the preferred M³D® apparatus ofthe present invention, configured for pneumatic atomization.

FIG. 2 is a graph showing the relationship between sheath gas pressureand total gas flow rate.

FIG. 3a is a schematic of a cross section of a passive structure withterminations. The height of the structure is t₁.

FIG. 3b is a schematic of FIG. 3a , with an additively trimmed passivestructure. The height of the structure is t₂, where t₂>t₁.

FIG. 4 is a schematic showing that the rightmost resistor has a greaterresistance than the middle structure, by virtue of the greater length ofresistor material between the pads.

FIG. 5a is a schematic of a ladder resistor prior to direct write ofadditional passive structures.

FIG. 5b is a schematic of a ladder resistor showing how structures canbe added after the board has been processed and populated with othercomponents, thereby tuning a circuit after it is mostly complete.

FIG. 6 is a schematic of a passive structure written over the edge of atarget.

FIG. 7a is a schematic of a linear passive trace with terminatedresistors.

FIG. 7b is a schematic of a serpentine passive trace with terminatedresistors.

FIG. 8 is a schematic of a resistor embedded in a via between twocircuit layers.

FIG. 9 depicts a method for depositing a coating on the walls and bottomof a via.

FIGS. 10a-c are schematics using the M³D® process in a hybridadditive/subtractive technique to fabricate precision metal structuresusing an etch resist.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Best Modes for Carrying Out theInvention

Introduction and General Description

The M³D® process is an additive direct printing technology that operatesin an ambient environment, and eliminates the need for lithographic orvacuum deposition techniques. The method is capable of depositing apassive electronic component in a predetermined pattern, and usesaerodynamic focusing of an aerosol stream to deposit patterns onto aplanar or non-planar target without the use of masks or modifiedenvironments. The M³D® method is compatible with commercial thick filmand polymer thick film paste compositions, and may also be used withliquid precursor-based formulations, particle-based formulations, andformulations consisting of a combination of particles and liquidprecursors. The method is also capable of depositing multipleformulations onto the same target layer. This capability enables directdeposition of resistive structures with a large range of resistancevalues—ranging from under 50 Ω/square to over 500 KΩ/square—onto thesame layer.

The M³D® method is capable of blending different formulations, forexample one low-value and one high-value composition, in-transit, in amethod in which multiple atomizers are preferably used to aerosolize thetwo compositions. The formulations are preferably deposited through asingle deposition head, and blending may occur during aerosol transport,or when the aerosol droplets combine on the target. This method allowsfor automated tailoring of a formulation, allowing for the resistivity,or other electrical, thermal, optical, or chemical property of thedeposit, to be continuously varied from the low value to the high value.The blending process can also be applied to pastes, inks, various fluids(including, but not limited to, chemical precursor solutions, particlesuspensions of electronic, optical, biological and bio-compatiblematerials, adhesives), and combinations thereof.

As used throughout the specification and claims, “passive structure”means a structure having a desired electrical, magnetic, or otherproperty, including but not limited to a conductor, resistor, capacitor,inductor, insulator, dielectric, suppressor, filter, varistor,ferromagnet, adhesive, and the like.

The M³D™ process preferably deposits material in an aerosolized form.Aerosolization of most particle suspensions is preferably performedusing a pneumatic device, such as a nebulizer, however ultrasonicaerosolization may be used for particle suspensions consisting of smallparticles or low-density particles. In this case, the solid particlesmay be suspended in water or an organic solvent and additives thatmaintain the suspension. The two atomization methods allow for thegeneration of droplets or droplet/particles with sizes typically in, butnot limited to, the 1-5 micron size range.

Ultrasonically aerosolized compositions typically have viscositiesranging from 1-10 cP. Precursor and precursor/particle compositionstypically have viscosities of 10-100 cP, and are preferably aerosolizedpneumatically. Compositions with viscosities of 100-1000 cP are alsopreferably pneumatically aerosolized. Using a suitable diluent,compositions with viscosities greater than 1000 cP may be modified to aviscosity suitable for pneumatic aerosolization.

The preferred apparatus of the present invention, which is capable ofdepositing passive structures having tolerances below 5% with extendedruntimes, is shown in FIG. 1a . FIG. 1b shows the M³D® apparatusconfigured for pneumatic atomization, and details the most generalembodiment of the apparatus. An inert carrier gas or carrier fluid ispreferably used to deliver the aerosolized sample to the depositionmodule. In the case of ultrasonic atomization, the aerosol-laden carriergas preferably enters the deposition head immediately after theaerosolization process. The carrier gas may comprise compressed air, aninert gas (which may comprise a solvent vapor), or a mixture of both.The pneumatic aerosolization process requires a carrier gas flow ratethat preferably exceeds the maximum allowable gas flow rate throughdeposition head 22. To enable the use of large carrier gas flow rates(for example, approximately 0.2 to 2 liter/min), a virtual impactor ispreferably used to reduce the flowrate of the carrier gas, withoutappreciable loss of particles or droplets. The number of stages used inthe virtual impactor may vary, and is dependent on the amount of carriergas that must be removed. The stream is introduced into the M³D®deposition head, where an annular flow is developed, consisting of aninner aerosol stream surrounded by a sheath gas. The co-flowingconfiguration is capable of focusing the aerosol stream to approximatelyone-tenth the size of the orifice diameter.

When fabricating passive structures using an annular flow, the aerosolstream preferably enters through ports mounted on deposition head 22 andis directed towards the orifice. Aerosol carrier gas flow controller 10preferably controls the mass throughput. Inside the deposition head, theaerosol stream is preferably initially collimated by passing through amillimeter-size orifice. The emergent particle stream is then combinedwith a sheath gas or fluid, forming an annular distribution consistingof an inner aerosol-laden carrier gas and an outer sheath gas or fluid.The sheath gas most commonly comprises compressed air or an inert gas,where one or both may contain a modified solvent vapor content. Thesheath gas enters through the sheath air inlet below the aerosol inletand forms an annular flow with the aerosol stream. Gas flow controller12 preferably controls the sheath gas. The combined streams exit thechamber through an orifice directed at target 28. This annular flowfocuses the aerosol stream onto target 28 and allows for deposition offeatures with dimensions as small as 10 microns or lower. The purpose ofthe sheath gas is to form a boundary layer that both focuses the aerosolstream and prevents particles from depositing onto the orifice wall.This shielding effect minimizes clogging of the orifices.

The diameter of the emerging stream (and therefore the linewidth of thedeposit) is controlled by the orifice size, the ratio of sheath gas flowrate to carrier gas flow rate, and the spacing between the orifice andtarget 28. In a typical configuration, target 28 is attached to a platenthat moves in two orthogonal directions under computer control via X-Ylinear stages, so that intricate geometries may be deposited. Analternate configuration allows for deposition head 22 to move in twoorthogonal directions while maintaining target 28 in a fixed position.Yet another configuration allows for movement of deposition head 22 inone direction, while target 28 moves in a direction orthogonal to thatof deposition head 22. The process also enables the deposition ofthree-dimensional structures.

In the M³D® method, once the sheath gas is combined with the aerosolstream, the flow does not need to pass through more than one orifice inorder to deposit sub-millimeter linewidths. In the deposition of a10-micron line, the M³D® method typically achieves a flow diameterconstriction of approximately 250, and may be capable of constrictionsin excess of 1000, for this “single-stage” deposition. No axialconstrictors are used, and the flows typically do not reach supersonicflow velocities, thus preventing the formation of turbulent flow, whichcould potentially lead to a complete constriction of the flow.

Aerosolization and Virtual Impaction

In the preferred operation of the system of the present inventiondetailed in FIG. 1a , Collison-type pneumatic atomizer 32 aerosolizesthe material in the sample vial. The aerosol-laden gas stream isdelivered to cross 30 that bridges atomizer 32, deposition head 22,exhaust flow controller 34, and pressure sensing transducer 36. Cross 30is preferably configured so that the aerosol flow inlet is opposite theaerosol flow outlet. The outlet is connected to the M³D® depositionhead. Excess carrier gas is preferably exhausted from the system 90°from the aerosol inlet/outlet line of travel. Mass flow controller 34 ispreferably used to control the amount of gas that is exhausted from thesystem. Controlling the exhaust flow using a flow controller increasesthe precision of the deposition process by aiding in the control of themass flux of the material that passes to the deposition head.

In an alternative embodiment, the atomizer is located directly adjacentto the virtual impactor. Positioning the virtual impactor near thepneumatic atomizer output results in the deposition of larger droplets,since the aerosol ultimately spends less time in transit from theatomizer to the target, and undergoes reduced evaporation. Thedeposition of larger droplets can produce a considerable effect on thecharacteristics of the deposited structure. In general, depositedstructures formed from larger droplets show less particle overspray andimproved edge definition when compared with structures deposited withsmall to moderate size droplets. The atomizer is optionally agitated toprevent material agglomeration.

Typically the carrier gas flowrate needed for pneumatic atomization mustbe reduced after the aerosol is generated, in order for the aerosolstream to be introduced into the deposition head. The required reductionin carrier gas flowrate—from as much as 2 L/min to as little as 10ml/min—is preferably accomplished using a virtual impactor. However, theuse of a virtual impactor may cause the system to be prone to clogging,decreasing the operating time of the apparatus to as little as severalminutes, while undesirably decreasing the tolerance of the depositedstructure. For example, the apparatus of FIG. 1b may depositcarbon-based resistors for as little as 15 minutes before failure, witha tolerance in the resistance values of as much as 30%. The apparatus ofFIG. 1a , contrastingly, replaces the standard M³D® virtual impactorwith cross 30 that exhausts excess carrier gas from the system, whileminimizing the loss of particles and buildup of particles with thesystem. Cross 30 acts as a virtual impactor with considerably larger jetand collector orifice diameters than those used with the standardimpactor. The use of larger jet and collector orifice diameters mayincrease the amount of material that flows through the virtual impactorminor flow axis, while minimizing the accumulation of material on theinterior of the device.

Leak/Clog Sensor

The present invention preferably uses a leak/clog sensor comprisingpressure transducers to monitor the pressure developed at the atomizergas inlet and at the sheath gas inlet. In normal operation, the pressuredeveloped within the system is related to the total gas flow ratethrough the system, and can be calculated using a second-orderpolynomial equation. A plot of pressure versus total flow through thesystem is shown in FIG. 2. If the system pressure is higher than thepressure predicted by the curve of FIG. 2, a non-ideal flow may havedeveloped within the system as a result of material accumulation. If thepressure is too low, a system leak is present, and material depositionmay be inhibited or stopped entirely. The second order polynomialequation of the curve representing normal operation is of the form:P=M ₀ +M ₁ Q+M ₂ Q ²where P is the sheath gas pressure and Q is the total flow rate. Thetotal flow rate through the system is given by:Q _(ultrasonic) =F _(sheath) +F _(ultrasonic)Q _(pneumatic) =F _(sheath) +F _(pneumatic) −F _(exhaust)where F is the device flow rate. The coefficients M₀, M₁, and M₂ areconstants for each deposition tip diameter, but are variable withrespect to atmospheric pressure.

The leak/clog sensor provides a valuable system diagnostic that canallow for continuous manual or automated monitoring and control of thesystem. When operating in an unassisted mode, the system may bemonitored for clogs, and automatically purged when an increase inpressure beyond a predetermined value is detected.

Mist Sensing

Quantitative measurement of the amount of aerosol generated by theatomizer units is critical for extended manual or automated operation ofthe M³D® system. Maintenance of a constant mist density allows forprecision deposition, since the mass flux of aerosolized materialdelivered to the target can be monitored and controlled.

The system of the present invention preferably utilizes a mist sensor,which preferably comprises a visible wavelength laser whose beam passedthrough the aerosol outlet tube of the atomizer unit. The beam ispreferably oriented perpendicular to the axis of the tube, and siliconphotodiodes are preferably positioned adjacent to the tube on an axisperpendicular to both the axes of the tube and the laser. As the laserinteracts with the mist flowing through the tube, light is scatteredthrough a wide angle. The energy detected by the photodiodes isproportional to the aerosol density of the mist flow. As the mist flowrate increases, the photodiode output increases until a state ofsaturation is reached, at which the photodiode output becomes constant.A saturated mist level condition is preferred for constant mist output,so that a constant photodiode output indicates an optimum operatingcondition.

In a feedback control loop, the output of the photodiodes is monitoredand can be used to determine the input power to the ultrasonic atomizertransducer.

Processing The aerosolized material compositions may be processedin-flight—during transport to the deposition head 22 (pre-processing)—oronce deposited on the target 28 (post-processing). Pre-processing mayinclude, but is not limited to, humidifying or drying the aerosolcarrier gas or the sheath gas. The humidification process may beaccomplished by introducing aerosolized droplets and/or vapor into thecarrier gas flow. The evaporation process is preferably accomplishedusing a heating assembly to evaporate one or more of the solvent andadditives.

Post-processing may include, but is not limited to using one or acombination of the following processes: (1) thermally heating thedeposited feature, (2) subjecting the deposited feature to a reducedpressure atmosphere, or (3) irradiating the feature with electromagneticradiation. Post-processing of passive structures generally requirestemperatures ranging from approximately 25 to 1000° C. Depositsrequiring solvent evaporation or cross-linking are typically processedat temperatures of approximately 25 to 250° C. Precursor ornanoparticle-based deposits typically require processing temperatures ofapproximately 75 to 600° C., while commercial fireable pastes requiremore conventional firing temperatures of approximately 450 to 1000° C.Commercial polymer thick film pastes are typically processed attemperatures of approximately 25° to 250° C. Post-processing mayoptionally take place in an oxidizing environment or a reducingenvironment. Subjecting the deposit to a reduced pressure environmentbefore or during the heating step, in order to aid in the removal ofsolvents and other volatile additives, may facilitate processing ofpassive structures on heat-sensitive targets.

Two preferred methods of reaching the required processing temperaturesare by heating the deposit and target on a heated platen or in a furnace(thermal processing), or by irradiating the feature with laserradiation. Laser heating of the deposit allows for densification oftraditional thick film pastes on heat-sensitive targets. Laserphotochemical processing has also been used to decompose liquidprecursors to form mid to high-range resistors, low to mid-rangedielectric films, and highly conductive metal. Laser processing mayoptionally be performed simultaneously with deposition. Simultaneousdeposition and processing can be used to deposit structures withthicknesses greater than several microns, or to build three-dimensionalstructures. More details on laser processing may be found incommonly-owned U.S. patent application Ser. No. 10/952,108, entitled“Laser Processing For Heat-Sensitive Mesoscale Deposition”, filed onSep. 27, 2004, the specification and claims of which are incorporatedherein by reference.

Thermally processed structures have linewidths that are partiallydetermined by the deposition head and the deposition parameters, andhave a minimum linewidth of approximately 5 microns. The maximum singlepass linewidth is approximately 200 microns. Linewidths greater than 200microns may be obtained using a rastered deposition technique.Laser-processed lines may have linewidths ranging from approximately 1to 100 microns (for a structure deposited with a single pass).Linewidths greater than 100 microns may be obtained using a rasteredprocessing technique. In general, laser processing is used to densify orto convert films deposited on heat-sensitive targets, such as those withlow temperature thresholds of 400° C. or less, or when a linewidth ofless than approximately 5 microns is desired. Deposition of the aerosolstream and processing may occur simultaneously.

Types of Structures: Material Compositions

The present invention provides a method for precision fabrication ofpassive structures, wherein the material composition includes, but isnot limited to, liquid chemical precursors, inks, pastes, or anycombination thereof. Specifically, the present invention can depositelectronic materials including but not limited to conductors, resistors,dielectrics, and ferromagnetic materials. Metal systems include, but arenot limited to, silver, copper, gold, platinum, and palladium, which maybe in commercially available paste form. Resistor compositions include,but are not limited to, systems composed of silver/glass, ruthenates,polymer thick films formulations, and carbon-based formulations.Formulations for deposition of capacitive structures include, but arenot limited to, barium titanate, barium strontium titanate, aluminumoxide, and tantalum oxide. Inductive structures have been depositedusing a manganese/zinc ferrite formulation blended with low-meltingtemperature glass particles. The present invention can also blend twouv-curable inks to produce a final composition with a targetedcharacteristic, such as a specific refractive index.

A precursor is a chemical formulation consisting of a solute or solutesdissolved in a suitable solvent. The system may also contain additivesthat alter the fluid, chemical, physical, or optical properties of thesolution. Inks may be comprised of particles, including but not limitedto metal nanoparticles or metal nanoparticles with glass inclusions, ofan electronic material suspended in a fluid medium. Depositable pastesinclude, but are not limited to, commercially available pasteformulations for conductive, resistive, dielectric, and inductivesystems. The present invention can also deposit commercially availableadhesive pastes.

Resistors

A silver/glass resistor formulation may be composed of a liquidmolecular precursor for silver, along with a suspension of glassparticles, or silver and glass particles, or silver particles in aliquid precursor for glass. A ruthenate system may be comprised ofconductive ruthenium oxide particles and insulating glass particles,ruthenium oxide particles in a precursor for glass, or a combination ofa ruthenium oxide precursor and a precursor for glass or an insulatingmedium. Precursor compositions and some precursor/particle compositionsmay have viscosities of approximately 10 to 100 cP, and may beaerosolized ultrasonically. Resistor pastes may be comprised of eitheror both of ruthenates, polymer thick film formulations, or carbon-basedformulations. Commercially available ruthenate pastes, typicallyconsisting of ruthenium oxide and glass particles, having viscosities of1000 cP or greater, may be diluted with a suitable solvent such asterpineol to a viscosity of 1000 cP or less. Polymer thick film pastesmay also be diluted in a suitable solvent to a similar viscosity, sothat pneumatic aerosolization and flow-guidance is enabled. Similarly,carbon-based pastes can be diluted with a solvent such as butyl carbitolto a viscosity of approximately 1000 cP or less. Therefore, manycommercial paste compositions with viscosities greater than 1000 cP maybe modified and deposited using the M³D® process.

Resistors: Range of Resistance, Repeatability, and TemperatureCoefficient of Resistance

The resistive structures deposited using the M³D® process may comprise aresistance spanning approximately six orders of magnitude, from 1 ohm to1 Mohm. This range of resistance values may be obtained by depositingthe appropriate material with the appropriate geometricalcross-sectional area. The tolerance or variance of the resistancevalues—defined as the ratio of the difference in the resistance value ofthe highest and lowest passive structure and the average resistancevalue, for a set of deposits—may be as low as 2 percent. The temperaturecoefficient of resistance (TCR) for Ag/glass and ruthenate structuresmay range from approximately ±50 to ±100 ppm.

Geometry

The present method is capable of producing a specific electronic,optical, physical, or chemical value of a structure by controlling thegeometry of the deposit. For example, properties of a structure can bealtered by controlling the cross-sectional area of the structure, asshown in FIGS. 3a and 3b . Resistance values may be altered by addingmaterial to an existing trace, thereby increasing the cross sectionalarea of the total trace, thus decreasing the resistance value asmaterial is added to the existing trace. This method is analogous tocommonly used laser trimming methods, however material is added ratherthan removed. The additively trimmed passive trace 38 is deposited ontothe existing passive trace 40. As a further example, a specific valuemay be obtained by controlling the length of a deposited structure; asshown in FIG. 4, the rightmost resistor has a greater resistance thanthe middle structure, by virtue of the greater length of resistormaterial between the contact pads. The method of the present inventionmay also be used to add material to a set of traces or between one ormore sets of contact pads 42 connected to a pre-existing electroniccircuit, as shown in FIGS. 5a and 5b . Ladder passive traces 44 a-b areadded to existing passive trace 40. This method enables tuning of thecircuit to a specific response or characteristic value. The method isalso capable of creating passive structures between layers of circuitryby making passive connections in vias, or by wrapping resistor material46 around the edge of circuit layers, as shown in FIG. 6.

The passive structures deposited using the M³D® process of the presentinvention typically have linear geometries, such as the linear passivetrace 48 shown in FIG. 7a . Other geometries include, but are notlimited to, serpentine 50 (as shown in FIG. 7b ), spiral, and helicalpatterns. Linewidths of deposited resistor material typically range fromapproximately 10 to 200 microns, but could be greater or lower.Linewidths greater than 200 microns may be obtained by depositingmaterial in a rastered fashion. The thickness of the deposited film mayrange from a few hundred nanometers to several microns.

Via Filling

The M³D® process can be used to fill vias, providing electricalinterconnectivity between adjacent layers of an electronic circuit. Thepresent invention allows for the precise, uniform deposition of anaerosolized material over an extended period of time, for example intovia holes.

FIG. 8 shows a resistive connection between different layers ofcircuitry. Conductive layers in a PCB (printed circuit board) aretypically connected by metal vias, however, the M³D® process also allowsfor deposition of resistive structures into vias. The resistive viaconfiguration is advantageous since, by moving the layer resistors intovias, additional space is provided on the surface of the circuit boardlayers.

FIG. 9 depicts a method for depositing a coating on the walls and bottomof a via. In FIG. 9a , via 60 is completely filled with ink 62 using theprocess of the present invention. As ink 62 dries, the solids 64 willadhere onto the walls and the bottom of the via, leaving the middle ofthe via hollow, as shown in FIG. 9b . Coating the wall with highlyconductive material results in a very useful structure, because most ofthe current in a via flows along the wall and not through the middle.

Dielectrics

In the case of fabrication of dielectric structures, an ink can becomprised of a precursor for an insulator, such as polyimide, while apaste may be a formulation containing dielectric particles and lowmelting temperature glass inclusions. The precision deposition offeredby the present invention is critical to fabrication of high tolerancecapacitors, since the thickness and uniformity of a capacitive filmdetermines the capacitance and the performance of the capacitor. Low-kdielectric materials such as glass and polyimide have been deposited fordielectric layers in capacitor applications, and as insulation orpassivation layers deposited to isolate electronic components. Mid-k andhigh-k dielectrics such as barium titanate can be deposited forcapacitor applications.

Etch Resist

The present embodiment of the M³D® process may be used in a hybridadditive/subtractive technique to fabricate precision metal structuresusing an etch resist. Etch resist 70 is preferably atomized anddeposited through the deposition head onto metal layer 72, as shown inFIG. 10a . A subtractive technique, for example etching, is then used toremove the exposed metal, FIG. 10b . In the last step, the etch resistis removed, leaving metal structure 74 on the underlying substrate, FIG.10c . The additive/subtractive etch resist process can be used todeposit reactive metals such as copper.

Targets

Targets suitable for direct write of passive structures using the M³D®process include, but are not limited to, polyimide, FR4, alumina, glass,zirconia, and silicon. Processing of resistor formulations on polyimide,FR4, and other targets with low temperature damage thresholds, i.e.damage thresholds of approximately 400° C. or less, generally requireslaser heating to obtain suitable densification. Laser photochemicalprocessing may be used to direct write mid to high range resistormaterials such as strontium ruthenate on polyimide.

Applications

Applications enabled by fabrication of passive structures using the M³D®process include, but are not limited to, direct write resistors forelectronic circuits, heating elements, thermistors, and strain gauges.The structures may be printed on the more conventional high-temperaturetargets such as alumina and zirconia, but may also be printed onheat-sensitive targets such as polyimide and FR4. The M³D® process mayalso be used to print embedded passive structures onto pre-existingcircuit boards, onto planar or non-planar surfaces, and into viasconnecting several layers of a three-dimensional electronic circuit.Other applications include, but are not limited to, blending passiveelement formulations to produce a deposited structure with a specificphysical, optical, electrical, or chemical property; repair of passivestructures on pre-populated circuit boards; and deposition of passivestructures onto pre-populated targets for the purpose of altering thephysical, optical, electrical, or chemical performance of a system. Thepresent invention enables the above applications with tolerances inphysical or electrical properties of 5% or less.

Although the present invention has been described in detail withreference to particular preferred and alternative embodiments, personspossessing ordinary skill in the art to which this invention pertainswill appreciate that various modifications and enhancements may be madewithout departing from the spirit and scope of the Claims that follow,and that other embodiments can achieve the same results. The variousconfigurations that have been disclosed above are intended to educatethe reader about preferred and alternative embodiments, and are notintended to constrain the limits of the invention or the scope of theClaims. Variations and modifications of the present invention will beobvious to those skilled in the art and it is intended to cover all suchmodifications and equivalents. The entire disclosures of all patents andpublications cited above are hereby incorporated by reference.

What is claimed is:
 1. A maskless method of depositing a via, the methodcomprising the steps of: aerosolizing a material; focusing theaerosolized material into a hole in an intermediate circuit layer havinga top surface, the hole comprising one or more side walls connected tothe to surface; depositing the aerosolized material within the hole sothat the aerosolized material is confined within the one or moresidewalls, but not deposited on the top surface; and processing thedeposited material in the hole to form a passive via; wherein thepassive via is electrically conductive.
 2. The method of claim 1 whereinthe passive via comprises a resistive element or a capacitive element.3. The method of claim 1 wherein the passive via is orientedsubstantially perpendicular to the layers.
 4. The method of claim 1wherein the material is deposited into the hole.
 5. The method of claim1 wherein the passive via comprises a highly conductive material.
 6. Themethod of claim 5 wherein the highly conductive material comprises ametal.
 7. The method of claim 1 wherein the hole is less thanapproximately 200 microns in size.
 8. The method of claim 7 wherein thehole is less than approximately 100 microns in size.
 9. The method ofclaim 8 wherein the hole is approximately 5 microns in size.
 10. Themethod of claim 8 wherein the hole is less than approximately 5 micronsin size.